1. Field of the Invention
The invention concerns a counterflow leak-detector unit with an inlet to which a test specimen, or a chamber housing a test specimen, is connected and with a test-gas detector to which is connected a high-vacuum pump which produces the necessary pressure in the detector, the inlet to the leak-detector unit being connected to an intermediate line in the high-vacuum pump.
2. Discussion of the Prior Art
A leak-detector of this kind is known from DE-PS 31 24 205. The stage on the high-vacuum side with respect to the intermediate line is designed as a counterflow stage and has a relatively low compression for light gases, in particular for helium which is generally employed. The pump stage (pre-stage) on the forevacuum side with respect to the intermediate line serves--besides a forevacuum pump--to evacuate the specimen or the specimen chamber, so that relatively fast cycle times can be achieved. The pre-requisite for this is, that the pre-stage has a relatively high pumping speed. During the leak detection process the test gases are also admitted into the intermediate line of the high-vacuum pump. Due to the relatively high pumping speed of the pre-stage, sensitivity of the leak detection process is limited.
A counterflow leak-detector of the aforementioned kind is known from European Patent Publication 268 777, in which the inlet of the leak-detector unit is directly connectable to the test gas detector, i.e. to the inlet side of the high-vacuum pump. Due to this measure it is basically possible to increase sensitivity; but in practice this solution can hardly be employed. The test gas detector is commonly a highly sensitive mass spectrometer which is set to the mass of the test gas, and which requires an operating pressure of less then 10.sup.-4 mbar. Owing to the direct connection of the specimen to the test gas detector there exists the danger of an inadmissibly high pressure increase, which gives rise to faulty measurements or misinterpretations.